Plasma processing is widely used in semiconductor manufacturing apparatuses, e.g., a surface treating apparatus and an etching apparatus. In the plasma processing, a substrate mounting device for mounting a target substrate such as a wafer or the like is provided within a processing chamber. The substrate mounting device includes, e.g., as shown in FIG. 5, a mounting table (wafer chuck) 2 for mounting a wafer 1 and a focus ring 3 arranged in an outer peripheral portion of the mounting table 2. An electrostatic chuck is commonly used as the wafer chuck.
When the wafer 1 is plasma processed in a processing chamber, the wafer 1 is placed on the wafer chuck 2 and then is fixed in place while the processing chamber is kept at a predetermined vacuum level (in case of using, e.g., an electrostatic chuck, the wafer is electrostatically attracted thereto by applying a DC voltage to a dielectric member of a mounting surface). Then, a high frequency voltage is applied to the wafer chuck 2 to generate a plasma in the processing chamber.
Since the temperature of the wafer 1 is increased by the plasma processing, the wafer 1 is cooled to maintain at a desired temperature by means of a cooling mechanism provided in the wafer chuck 2. In this process, a frequently applied method is to increase the heat conduction rate between the wafer 1 and the wafer chuck 2, wherein a helium gas of high heat conductivity is allowed to flow from the top surface of the wafer chuck toward the rear surface of the wafer 1.
In this regard, the focus ring 3 may be provided to reduce the discontinuity in the plasma in the outer peripheral portion of the target substrate, so that uniformity in the plasma processing over the entire surface of the target substrate is improved (see, e.g., Japanese Patent Laid-open Application Nos. 2002-016126, 2002-033376 and 2006-220461).
Since the conventional target substrate mounting device is of a structure wherein the focus ring 3 is merely placed on the wafer chuck 2, micro-sized fine gaps are formed between the wafer chuck 2 and the focus ring 3. Thus, the focus ring 3 fails to make a perfectly tight contact with the wafer chuck 2 to thereby allow air to be left in the fine gaps, causing that the focus ring 3 cannot be sufficiently cooled.
In other words, although the portion of the wafer other than the peripheral portion is sufficiently cooled by the cooling effect of the cooling mechanism and the helium gas, the temperature of the focus ring 3 is increased in the outer peripheral portion of the wafer. And then, the heat of the focus ring is transferred to the outer peripheral portion of the wafer to thereby increase the temperature of thereof. The etching property of the outer peripheral portion of the wafer becomes worsened by the influence of the increased temperature. This causes a problem in that the release property of etching openings (the properties with which the wafer can be reliably etched into a specified depth in the etching process) is deteriorated and the aspect ratio of the etching is decreased.
As a countermeasure for preventing the temperature increase in the focus ring, JP2002-16126A discloses a target object mounting device in which a heat transfer medium is interposed between a mounting table and a focus ring, wherein the target object mounting device includes a pressing means for pressing and fixing the focus ring to the mounting table. JP2002-33376A discloses a target object mounting device that includes an electrostatic adsorption means for attracting a focus ring.
However, the target object mounting devices disclosed in the above patent documents need to be provided with a drive mechanism for physically pressing the focus ring against the mounting table or an electrode for electrostatically to the focus ring. This is problematic in that the mounting devices become structurally complicated and suffer from increased installation costs. Another problem is that the drive mechanism and the electrode for electrostatic adsorption may cause abnormal plasma discharge and generate particles or dust.